This invention relates to a cathode of a gas discharge tube which is utilized as an illuminant.
A conventional gas discharge tube will be described hereinafter, particularly the structure and drawbacks of a deuterium gas discharge tube used as an illuminant for a measuring instrument. In a deuterium gas discharge tube operating at a pressure of several torrs of deuterium gas, the anode column of an arc which emits UV rays is used as an illuminant for optical instruments such as a spectroscope. This type of tube stably emits contiguous spectral lines in the UV portion, of the spectrum and is used as an illuminant to provide UV rays.
The structure of such a typical deuterium gas discharge tube is described with reference to FIGS. 1 and 2. FIG. 1 shows a sectional view of a conventional deuterium gas discharge tube. An anode 3, a cathode 4, and a shielding electrode 5 are provided in a vacuum-sealed envelope 2 having a window 1 through which the UV rays can pass. The shielding electrode 5 surrounds the anode 3 and cathode 4, and a small hole 6 is bored through a separator between the anode 3 and the window 1 through which the UV rays can pass. The cathode 4 is set off the line leading from the anode 3 to the small hole 6. Another hole 7 is bored through another separator between the above line and the cathode 4. When the cathode 4 is heated and simultaneously a voltage is applied to the anode 3, an arc occurs in a space covering the anode 3, small holes 6 and 7, and cathode 4. The anode column shrinks at the small hole 6 thereby becoming a bright spot on the front side of the small hole 6.
If such a conventional deuterium gas discharge tube is used as the illuminant for liquid chromatography, fluctuations in intensity directly affect the measured values. That is, the resolving power (accuracy) is determined by fluctuations during the measurement. This is the reason that a deuterium gas discharge tube with a stability as high as possible has been expected. Fluctuations in intensity may mainly be caused by the flicker and shot noise generated by the cathode. The flicker noise may be caused by a small amount of structural disorder at the cathode surface.
Since the cathode of a deuterium gas discharge tube is exposed to an arc, cathode material may be sputtered by ions. Cathode material meet firmly be fastened to the support so as to avoid sputtering of the cathode. Cathode material which is only coated on the metal surface is not satisfactory, and in this case, a solid-state cathode must be used. Thus, in the conventional cathode, the cathode material has been placed within the space around a small-diameter spiral coil 9 formed to build a double coil 8 by winding a tungsten wire filament. (Hereinafter the coils 9 and 8 are called the primary and secondary coils, respectively.) By the way, a paste made from a powder of carbonates, i.e., barium carbonate, strontium carbonate, and calcium carbonate, and a binder composed of nitrocellulose dipped in organic solvent, i.e., butyl acetate are used to fabricate the cathode. Therefore, the double coil is fully stretched by drawing one end of the double coil from the other, and then coating the coil with a suitable quantity of paste. The double coil thereafter is restored to the original state and excessive paste protruding from the primary coil is rubbed off. If this conventional method mentioned above is used for fabricating the cathode by depositing cathode material on the primary coil of the double coil, voids can occur in the deposited cathode material due to its high viscosity. Furthermore, it is difficult to fasten the double coil because of the large elastic deformation in the double coil. It is also difficult to remove excessive cathode material, without removing that covering the gaps between the spiral windings, from the coil surface so as to keep the double coil surface flat. When a cathode fabricated in this way is fastened, by welding its both ends, to the support in a deuterium gas discharge tube and heated by a current flowing through the double coil, both nitrocellulose and organic solvent are removed by evaporation or vaporization, from deposited material and the carbonate is changed to the oxide which finally acts as a cathode material. The oxide is a hard lump of material which tends to generate cracks when a thermal shock caused by applying repetitive heat cycles between high and low temperatures is imposed on the cathode material, and the cracked cathode material tends to drop off during vibration or mechanical shock. A double coil having both ends fastened to the support may be deformed by mechanical expansion when heated by a current flowing through the double coil in a deuterium gas discharge tube, the cracks are made larger by the pressure applied on the cathode material. Discharging occurs at a spot on the cathode surface. If a new cathode surface appears when the cathode surface partly cracks or drops off, discharging goes to a point on the new cathode surface. This is because the new cathode surface provides an emissivity higher than the remaining part. The beam intensity of the UV rays changes before and after the discharging spot moves. This type of deformation may be a cause in the cathode for flicker noise, thereby making the cathode unstable.